1. Field of the Invention
The present invention relates to a composition for forming an insulating film on an electromagnetic steel plate, which can be applied on such a steel plate as can reduced or limit foaming thereon but having improved workability, and further excels in film properties as represented in terms of insulating properties, punchability and space factor, and to a method for making the same.
2. Statement of the Prior Art
In the prior art, a number of techniques have been proposed in relation to the methods for forming insulating films on the surfaces of electromagnetic steel plates or the compositions for forming such insulating films. According to the prior art, inorganic films are primarily formed using the compositions for forming insulating films (hereinafter often referred to as the film or films), which are composed mainly of chromates or phosphates or a combination thereof and, in some efforts, the film properties of the films have been improved by various methods involving the addition of metal oxides such as MgO, ZnO and CaO etc. or whatever which enhance their insulating properties by reacting with free acids present in the film-forming compositions, the addition of small quantities of glycerin or saccharides for promoting the aforesaid reactins and forming oxides insoluble in water, organic solvents or whatever, and the addition of boric acid, water glass, condensed phosphates, silica or whatever. In the present disclosure, the aqueous solutions of the inorganic film-formable substances composed mainly of chromates or phosphates, as mentioned above, may hereinafter be called the inorganic film-forming composition(s). However, the inorganic films, a most part of which is occupied and consituted by such inorganics, have had the disadvantages that they are poor in punchability and adhesiveness during processing, although they have been found to excel in insulating properties.
In recent years, increased weight has been placed on the punchability of electromagnetic steel plates. Methods attempted from this point-of-view have included the formation of organic films on the aforesaid inorganic films and the application of the film-forming composition containing a mixture of an inorganic film-forming element (hereinafter may be referred to as the inroganic component) with an organic film-formable resin (which may hereinafter be called either as the organic component or simply as the resin), thereby forming an inorganic/organic combination film.
However, the former method should be carried out in a two coating processes, and is unavoidably rather costly.
In order to carry out the latter method, a mixed liquid is prepared by mixing the inorganic film-forming composition containing as the inorganic component a chromate or phosphate showing a strong oxidative effect with an aqueous emulsion in which the organic film-formable resin is emulsified and dispersed, and is used. However, this method has the following disadvantages.
First, the pot life is short. In other words, the emulsified dispersion of the resin is no unstable that the mixed liquid is gelled due to the agglomeration of the resin for several hours after mixing, and is often unusable for ordinary coating.
Second, since the mixed liquid contains an emulsifier or dispersant used for the preparation of the aqueous solution of the resin, it is foamed or subjected to increased viscosity when stirred during coating due to circulation by a pump, supply operation, rotation of the roll of a roll coater and the like, thus making it impossible to form a satisfactory film. Especially when foaming takes place, there are found crateriform pin holes or foamy defects in the resulting film, which not only give rise to a drop in corrosion resistance, but also lead to poor insulating performance, thus rendering it impossible to obtain a uniform and satisfactory film.